1. Field of the Invention
The present invention relates to an image processing technique, and more particularly, to a technique determining whether an input image is a 2:2 pull-down image.
2. Description of Related Art
Generally, a film source, such as a movie, includes 24 progressive frames per second. As for the National Television System Committee (NTSC) TV signal, for example, each signal includes 60 interlaced fields per second. As for the Phase Alternating Line (PAL) TV signal, for example, each signal includes 50 interlaced fields per second. Accordingly, in order to broadcast the film source to a TV set, the 24 progressive frames per second need to be converted to the 60 or 50 interlaced fields per second. It is a 3:2 pull-down scheme that is used in converting the signal to the NTSC TV signal, and a 2:2 pull-down scheme used in converting the signal to the PAL TV signal.
The image which is originally generated and taken as a progressive image such as animation or computer graphic (CG) includes 30 frames per second. In this case, the 2:2 pull-down scheme is also employed when the signal is converted into the NTSC TV signal as well.
Hereinafter, the image formed of the interlaced fields is called interlaced image, the interlaced image obtained by the 3:2 pull-down scheme based on the progressive image is called 3:2 pull-down image, and the interlaced image obtained by the 2:2 pull-down scheme based on the progressive image is called 2:2 pull-down image.
When the interlaced image is reproduced by an output device of progressive scheme such as liquid crystal television, Interlace to Progressive Conversion (IPC) needs to be performed in a reproducing side. In this case, it is possible to reproduce the interlaced image obtained by the pull-down scheme with high quality by determining whether the interlaced image is originally interlaced image or the interlaced image obtained by the pull-down scheme.
The 3:2 pull-down scheme converts two 24 Hz frames into five 60 Hz fields, for example. More specifically, one field is repeated to form three fields from a first frame of two continuous frames of the film source while two fields are formed from a second frame. The 3:2 pull-down scheme extracts from the first frame a top field having odd lines and a bottom field having even lines, and then re-extracts the top field. After that, the 3:2 pull-down scheme extracts another bottom field and another top field from the second frame.
When the 60 Hz field image such as NTSC scheme is reproduced by the progressive scheme, a reproducing side obtains the complete progressive frames before the 3:2 pull-down conversion, through a combination of the two fields of the interlaced fields if the reproducing side recognizes the input image as the 3:2 pull-down image. Accordingly, the reproducing side can obtain original images having a high quality which is the same as the image before the pull-down conversion (Japanese Unexamined Patent Application Publication No. 2004-40815).
The 2:2 pull-down scheme is a scheme of extracting two fields from one frame. More specifically, as shown in FIG. 18, the 2:2 pull-down scheme extracts from each frame of the progressive image a top field having odd lines first and then a bottom field having even lines. In FIG. 18, the shading part is the part omitted in the extracting operation.
When the interlaced image is reproduced by the progressive scheme, a reproducing side can obtain the image which is the same as the original progressive image through a combination of the top fields and the bottom fields as shown in the lower part in FIG. 18 if the reproducing side recognizes the input image as the 2:2 pull-down image. Accordingly, the reproducing quality can be increased.
There have been suggested various schemes for determining whether the input image is the 2:2 pull-down image.
For example, in a case of the PAL TV image which is not the 2:2 pull-down image, there is a time difference of 20 ms between the fields. Therefore, as shown in FIG. 19, the correlation is low between the two fields adjacent to each other on a time basis. On the other hand, in FIG. 20, the correlation between the fields adjacent to each other on the time basis is obtained based on the fact that the correlation between two fields (top field and bottom field) generated from one frame is higher than the correlation between the two fields and a field generated by a different frame, so as to determine the input image in which the correlation patterns of “high, low, high, low, . . . ” are repeated as the 2:2 pull-down image.
Further, Japanese Unexamined Patent Application Publication No. 2007-129400 discloses a technique of detecting a 2:2 pull-down image by dividing a field into a plurality of blocks and performing motion compensation to detect a most reliable motion vector between two fields of identical attribute continuous in each block, and comparing motion vector of continuous odd field and even field. The term “attribute” indicates whether the field is the top field or the bottom field; the odd fields have the same attribute and the even fields have the same attribute.